U.S. patent number 6,796,187 [Application Number 10/220,102] was granted by the patent office on 2004-09-28 for wireless multi-functional sensor platform, system containing same and method for its use.
This patent grant is currently assigned to The Johns Hopkins University. Invention is credited to Russell P. Cain, Bliss G. Carkhuff, Kenneth R. Grossman, Robert Osiander, Jane W. Spicer, Regaswamy Srinivasan, Francis B. Weiskopf, Jr..
United States Patent |
6,796,187 |
Srinivasan , et al. |
September 28, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Wireless multi-functional sensor platform, system containing same
and method for its use
Abstract
A multi-functional sensor system for simultaneously monitoring
various parameters such as the structural, chemical and
environmental conditions associated with a medium to be monitored,
e.g., bridges, high-rise buildings, pollution zones, is provided
wherein the system includes at least a plurality of wireless
multi-functional sensor platforms embedded in the medium in which
an interrogation unit transmits power and receives responses. Each
wireless multi-functional sensor platform includes multiple
channels for accommodating a plurality of sensor types to
simultaneously monitor the parameters associated with the medium.
Thus, the wireless sensor platforms are formed to include those
sensor types which are considered germane to the intended medium to
be monitored.
Inventors: |
Srinivasan; Regaswamy (Ellicott
City, MD), Osiander; Robert (Ellicott City, MD), Spicer;
Jane W. (Columbia, MD), Weiskopf, Jr.; Francis B.
(Catonsville, MD), Grossman; Kenneth R. (Olney, MD),
Cain; Russell P. (Columbia, MD), Carkhuff; Bliss G.
(Laurel, MD) |
Assignee: |
The Johns Hopkins University
(Baltimore, MD)
|
Family
ID: |
26943832 |
Appl.
No.: |
10/220,102 |
Filed: |
August 27, 2002 |
PCT
Filed: |
December 07, 2001 |
PCT No.: |
PCT/US01/46806 |
PCT
Pub. No.: |
WO02/46701 |
PCT
Pub. Date: |
June 13, 2002 |
Current U.S.
Class: |
73/784 |
Current CPC
Class: |
G01D
9/005 (20130101); G01D 21/02 (20130101); G01M
5/0008 (20130101); G01M 5/0033 (20130101); G01M
5/0083 (20130101); G01M 5/0091 (20130101); G01N
33/383 (20130101) |
Current International
Class: |
G01D
9/00 (20060101); G01D 21/02 (20060101); G01B
005/00 () |
Field of
Search: |
;73/784,801,802,803,805,804,811,778,786 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
3736873 |
|
May 1989 |
|
DE |
|
2342998 |
|
Apr 2000 |
|
GB |
|
07306299 |
|
Nov 1995 |
|
JP |
|
WO 00/50849 |
|
Aug 2000 |
|
WO |
|
Other References
PCT International Search Report PCT/US01/46806 Dated Jul. 12, 2001.
.
Embedding Sensor for Corrosion Measurement, SPIE Proceedings, vol.
3587, Non Destructive Evaluation of Bridges & Hgwys.III, p
16-27 by Kelly et al. .
Embeddable Microinstruments for Corrosion Monitoring Paper 97294,
Corrosion 97, Natl. Assn. of Corr. Eng., (NACE) 1997, By Kelly et
al..
|
Primary Examiner: Noori; Max
Attorney, Agent or Firm: Fasulo, II; Albert J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of prior filed, co-pending U.S.
provisional application serial Nos. 60/254,118, filed on Dec. 8,
2000 and 60/284,018, filed on Apr. 16, 2001.
Claims
What is claimed is:
1. A sensor system for monitoring of a medium, comprising: an
interrogation unit for generating an induction power field and for
receiving responses; and a plurality of wireless embedded sensor
platforms, each including a plurality of sensor types and powered
by said induction field, said platforms are disposed throughout
said medium, and wherein each of said plurality of sensor types
generates a respective output representative of said sensor type,
wherein each of said sensor platforms comprises: a sensor housing
having a cap portion and a base portion defining an interior
volume, said plurality of sensors; and sensing electronics enclosed
within said housing interior volume and operatively coupled to said
plurality of sensors, said sensing electronics comprising: a
processor coupled to said plurality of sensors for monitoring
outputs generated from said plurality of sensor types; and transmit
circuitry coupled to said processor for transmitting said outputs
representative of said sensor types.
2. The wireless sensor platform of claim 1, wherein said housing is
made from a ceramic material.
3. The wireless sensor platform of claim 1, wherein said sensing
electronics further includes means for being externally
powered.
4. The sensor system of claim 1, wherein each sensor platform is
made from a ceramic material.
5. The sensor system of claim 1, further comprising analog to
digital conversion means for converting an analog output of said
plurality of sensors to a digital output.
6. A sensor system for monitoring of a medium, comprising: an
interrogation unit for generating an induction power field and for
receiving responses; and a plurality of wireless embedded sensor
platforms, each including a plurality of sensor types and powered
by said induction field, said platforms are disposed throughout
said medium, and wherein each of said plurality of sensor types
generates a respective output representative of said sensor type,
further comprising a multi-layered substrate for mounting said
sensing electronics.
7. The wireless sensor platform of claim 1 or 6, wherein said
medium is selected from the group consisting of ground, asphalt,
composites, plastics, concrete and cement.
8. The wireless sensor platform of claim 1 or 6, wherein said
sensors sense a plurality of parameters selected from the group
consisting of temperature, conductivity, pH, magnetism, noise,
pressure, shock, strain, stress and vibration.
9. The sensor system of claim 6, wherein said multi-layered
substrate is constructed of tape dielectric materials and screen
printed thick-film conductor.
10. The wireless sensor platform of claim 1 or 6, wherein said
sensors sense a plurality of parameters indicating structural,
chemical and environmental conditions associated with the
medium.
11. The sensor system of claim 6, wherein said multi-layered
substrate is a low-temperature co-fired ceramic substrate.
12. The sensor system of claim 6, wherein one layer of said
multi-layered substrate defines a patterned transmission
antenna.
13. The sensor system of claim 6, wherein said plurality of
wireless embedded sensor platform are individually addressable.
14. A method for providing monitoring of a medium, comprising the
steps of: providing a plurality of wireless embedded sensor
platforms in said medium, wherein the medium is one of asphalt,
concrete and cement, said sensor platforms having a housing
defining an interior volume; and a plurality of sensors configured
for inserting within said interior volume and for monitoring a
plurality of parameters associated with said medium; embedding the
plurality of embedded wireless sensor platforms within the medium
to be monitored; generating a time varying magnetic field to said
plurality of embedded wireless sensor platforms; wirelessly
receiving the time varying magnetic field at said plurality of
embedded wireless sensor platforms; powering said plurality of
embedded wireless sensor platforms from the wirelessly received
time varying magnetic field; sensing a plurality of structural,
chemical and environmental conditions of the monitored medium from
said powered embedded wireless sensor platforms; wirelessly
transmitting said sensed plurality of structural, chemical and
environmental conditions of the monitored environment to a
receiving unit.
15. The method of claim 14, wherein the receiving unit is one of a
mobile interrogation unit, a hand-held unit and a stationary
unit.
16. The method of claim 14, wherein said receiving unit receives
said sensed plurality of structural, chemical and environmental
conditions while said receiving unit is in proximity with the
monitored medium.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present disclosure generally relates to a wireless
multi-functional sensor platform, a sensor system containing same
and method for its use. More particularly, the present disclosure
is directed to an in-situ multi-functional sensor system containing
a plurality of wireless multi-functional sensor platforms and
method for providing long-term monitoring of various parameters
associated with a medium, e.g., concrete, by embedding the sensor
system within the medium to predict the onset of degradation and
thus aid in the scheduling of maintenance, management and repair
thereof.
2. Description of the Related Art
In the United States, billions of dollars have been spent in the
construction of highways, freeways and their associated overpasses,
bridges and buildings. One of the most important problems facing
the nation is determining how to maintain the integrity of this
system of roads and other structures at an acceptable cost.
Obviously, it would be advantageous for practitioners in the art to
have the benefit of a permanent, early-warning system for detecting
structural degradation in the earliest stages.
One of the primary applications of this technology is in the area
of bridge-deck monitoring. Currently, bridge deck monitoring is
based on individual sensor measurements or periodic visual
inspection by trained personnel. This approach doesn't detect
bridge deck or foundation degradation until it has already reached
an advanced state. By this time, remedial actions are more
expensive than if the problem had been detected earlier. In
addition, significant degradation impacts repair schedules and
quality of service for the bridge.
Yet another problem associated with present day sensor systems for
use in bridge monitoring is that the sensors are not distributed
throughout the bridge deck. Instead, they are used only for
discrete measurements, mostly due to economic limitations.
Furthermore, the cost of making measurements employing present day
technology is high due to installation and monitoring
requirements.
Further problems associated with prior art solutions for bridge
monitoring is that recent research has focused on mechanical
sensing such as stress/strain and pressure. Sensors that are being
designed to address corrosion-related degradation are limited to
specific parameters such as, for example, chloride and temperature,
or gross measurements of physical properties such as
conductivity.
Thus, it would be particularly advantageous to employ sensors which
measure a multitude of parameters for various mediums that extend
beyond those described above. Such parameters may include those
related to the structural, chemical and/or environmental conditions
associated with a medium such as, for example, magnetism, noise,
pH, pressure, shock, strain, stress and vibration. Accordingly, a
need exists for a multi-functional sensor system for providing
long-term monitoring of a plurality of parameters of a medium to
preemptively detect the onset and degree of degradation. In this
manner, protective measures can be promptly taken to ensure that
the medium is properly maintained.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
early-warning multi-functional sensor system for monitoring a
plurality of parameters, e.g., structural, chemical and/or
environmental conditions, associated with a medium such that the
onset of degradation can be detected employing the multi-functional
sensor system.
It is a further object of the present invention to provide an
early-warning multi-functional sensor system that may be embedded
in a medium such as, for example, concrete buildings, bridges or in
contaminated ground zones, for long-term monitoring of the medium
to both detect the onset of degradation and to prevent or forestall
further degradation.
Yet another object of the present invention is to provide a
plurality of wireless multi-functional sensor platforms for use in
the early-warning sensor system that are compact in size,
relatively low in cost and are capable of being remotely powered to
facilitate their long term use such that numerous sensors may be
used in a single project, e.g., embedded in a reinforced concrete
bridge. The sensor platforms are designed to be powered and queried
remotely as often as required for use in measuring a plurality of
parameters of the medium in which they are embedded.
A further object of the present invention is to provide a plurality
of wireless multi-functional sensor platforms that are capable of
monitoring the medium in a nondestructive manner.
It is a further object of the present invention to provide a
plurality of wireless multi-functional sensor platforms which may
serve as an attachment base for supporting a plurality of sensor
types on each platform specifically selected for use in monitoring
a particular parameter associated with the medium to which the
sensor platforms are embedded in.
It is yet a further object of the present invention to provide a
plurality of wireless multi-functional sensor platforms that
exhibit extremely high reliability for a prolonged period, e.g., on
the order of several decades.
In keeping with these and other objects of the present invention,
an early-warning multi-functional sensor system and method for
using same are provided which includes a network of cost-effective,
embeddable, remotely powered, ultra-small, ruggedized and
long-lasting wireless multi-functional sensor platforms that are
impervious to harsh environmental conditions such as salt,
mechanical and thermal stress. The sensor platforms are
particularly suited for long-term field measurements of parameters
in a harsh environment. The sensor platforms are preferably
constructed from a housing material that is of low cost and
requires only standard automated machining, e.g., a ceramic
material.
Accordingly, the sensor platforms are multi-functional in that they
serve as platforms for attaching a multitude of sensor types (e.g.,
temperature, conductivity, pressure, pH, etc.) thereto for
monitoring various parameters specific to the medium to be
monitored. This capability of interchanging sensor types dependent
upon the particular medium makes the sensor system of the present
invention suitable for use in a wide variety of monitoring
situations. Thus, when the platforms are employed in the sensor
system, sensor platforms having a plurality of sensor types
attached thereto are distributed throughout the medium to be
monitored to acquire data directed to, for example, structural,
chemical and environmental data, associated with the medium. The
sensor system therefore advantageously provides an early warning
indication of the present state of the monitored medium to aid in
the medium's timely maintenance and/or repair.
According to one aspect of the present invention, the operation of
the sensor system includes disposing a plurality or network of
wireless multi-functional-sensor platforms throughout a medium or
zone in the medium to be monitored, with each of the sensors
generating an output. Sensor data can then be collected
periodically, via wireless means, which may be combined with
historical data for analysis to ascertain the health of the
medium.
Data collection is performed by an interrogation unit operable to
generate power to and receive responses from the plurality of
sensor platforms. In this regard, the data is collected in a
non-invasive manner without impact on the medium being
monitored.
In one exemplary application, the system of the invention involves
distributing the wireless multi-functional sensor platforms
approximately every two meters throughout a medium during or after
construction. Periodically, a field data acquisition system passes
over the network of sensors to infer the sub-surface environment.
The resulting data is then used to forecast potential problem
regions within the medium and measure the evolution of the
structural, chemical and environmental parameters of the medium
over time. As such, the sensor system provides an early warning
indication of potential and ongoing adverse structural, chemical
and environmental changes within the medium.
Other objects and advantages of the present invention will become
more fully apparent from the following, more detailed description
and the appended drawings, which illustrate several embodiments of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an exploded view of a wireless embedded sensor
platform, according to the present invention;
FIG. 2 illustrates a top view of the embedded patterned
transmission antenna 119, constructed as a layer of the ceramic cap
of the sensor platform of FIG. 1;
FIG. 3 illustrates a back or external view of the ceramic cap of
the sensor platform of FIG. 1;
FIG. 4a illustrates a first embodiment of sensor
electronics/circuitry contained within the sensor platform of FIG.
1;
FIG. 4b illustrates a second embodiment of sensor
electronics/circuitry contained within the sensor platform of FIG.
1;
FIG. 5 is a block diagram of an interrogation unit used in
accordance with the system of the present invention;
FIG. 6 is an illustration of an exemplary application of the
present invention directed to bridge monitoring; and
FIG. 7 is an illustration of a map generated as a product of using
the system of the present invention for the illustrative
bridge-modeling example of FIG. 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The sensor system of the present invention provides a long-term
monitoring capability of a harsh environment for various mediums in
need of monitoring. Such mediums include, but are not limited to,
asphalt, composites, plastics, cement, concrete, e.g., structures
such as buildings or bridges, apparatuses, e.g., heavy machinery,
and zones of interest, e.g., stream run-off, pollution or
contamination zones around areas such as, for example, storage
tanks, pipelines, bays and streams. In addition to the foregoing
mediums, the present invention will find general applicability to
any medium in which long-term monitoring of a harsh environment is
a requirement. For optimum results, however, the medium should be
relatively non-absorptive of the electromagnetic energy spectrum
used to interrogate the wireless embedded sensor platforms.
Accordingly, by employing the sensor system disclosed herein,
relevant parameters such as the aforementioned structural, chemical
and environmental conditions associated with the medium can be
monitored over an extended period of time, on the order of several
decades, to assess the state of health of the medium in order to
predict the onset or occurrence of degradation. Thus, the present
invention provides an early detection or warning capability of such
degradation in order to schedule corrective measures such as, for
example, maintenance and repair, in a timely manner rather than
having to incur more costly and time-consuming repair at a later
stage of degradation.
For example, in the case of a structure such as a bridge, the
system of the present invention allows authorities to routinely
perform an accurate and relevant assessment of structural integrity
to schedule maintenance and repair operations in a timely and more
cost effective manner. As a further example, in the case of a
pollution zone, the present invention provides authorities with a
capability for monitoring the extent and rate of the pollution zone
to prevent further degradation and to assess the effects of
remediation processes.
The sensor system of the present invention includes at least a
plurality of ultra-small, rugged, wireless sensor platforms that
are embedded in the medium to be monitored by strategically or
randomly placing the platforms throughout the medium. The wireless
sensor platforms are multi-functional in that they provide a
support base for the insertion of a multitude of sensor types
(e.g., pressure, conductivity, pH, stress, strain, etc.), which are
described hereinbelow, for measuring parameters of relevance to an
intended monitored medium. For example, in the case where stress,
temperature and pH are considered relevant parameters for a
particular monitored medium, then a stress sensor, a strain sensor
and a pH sensor will be implemented in the sensor platform having
three sensor channels for accommodating the three selected sensors
at the configuration stage. It is well within the scope of the
invention to construct a sensor platform that contains a greater
number of sensor channels for accommodating additional sensor
types.
A particular embodiment for the sensor system of the present
invention is that for sensing a change in the sub-surface
environment in bridges. The Federal Highway Administration has
identified over 170,000 US bridges in need of some substantial
repair, many of which were due to deck rebar corrosion. One of the
problems associated with such structures as bridges, high-rise
parking lots and large buildings has been the absence of precise
and quantifiable information regarding the corrosion state of the
rebars and the corresponding need for counter-corrosion measures.
The sensor system of the present invention provides a capability
for non-invasively monitoring the evolution of degradation over
time without impacting the monitored medium.
In addition to collecting relevant structural, chemical and
environmental data, the sensor system of the present invention can
advantageously collect supplemental data which results from the
interaction between the plurality of individual wireless embedded
sensor platforms. As one example, acoustic measurements may be
suitable for the detection of cracks in concrete. However, acoustic
measurements are somewhat limited due to the poor propagation of
acoustic waves in heterogeneous materials such as concrete. But as
the distance between sender and receiver is reduced, as in the case
of transmission between two embedded and proximally located sensor
platforms, such a measurement becomes feasible.
In a preferred embodiment, data specific to the structural,
chemical and environmental conditions associated with the medium is
collected periodically from a network of wireless embedded sensor
platforms by wirelessly interrogating the wireless platforms using,
for example, a field data acquisition system, which periodically
passes over the platforms to infer the sub-surface environment of
the medium. The resulting data can then be combined with historical
data, obtained from prior interrogations, to forecast potential
problem regions within the medium to enable prediction of expected
failures and to make necessary repairs in a timely manner.
The present invention also provides a safety factor as well as an
economic factor, and has application for degradation monitoring for
transportation safety relative to bridges, tunnels, underpasses,
overpasses, etc. Also, the invention has applications for
environmental monitoring, such as acid rain conditions and its
effect on degradation of structural materials, or other
industrially or urban-induced problems. Thus, the invention is also
an in-situ sensor system, employing a plurality of wireless
embedded sensor platforms to provide information regarding
environmental conditions.
Since the rates/modes of structural degradation on a given
structure are related to environmental conditions, such as low or
high pH, stress, temperature and vibration, the sensor system of
the present invention is well-suited as an "early warning" system
to flag the onset of electrical, chemical and structural
degradation. Thus, the network of wireless embedded sensor
platforms which simultaneously monitor the above and other
parameters provide both maintenance and safety information.
Although specific reference is made herein to embedded sensors,
those skilled in the art will readily appreciate that the term
"embedded" as used herein is intended to be interpreted in its
broadest sense to include, for example, sensors disposed on a
surface of or integrated with a medium. Further, it is to be
understood that the sensors may be distributed in any manner
throughout a medium so as to optimize the information provided
therefrom. By way of example, in the case of a concrete pylon, it
may be desirable to distribute the sensors with different densities
on the top and bottom of the concrete pylon where stresses may be
higher on the top of the pylon. In other applications, cost may be
a driving factor in the determination of the distribution and
densities of the sensors.
When embedding the wireless sensor platforms in the medium, the
platforms can be held in place by structural members such as, for
example, reinforcements or rebars using a holder or attached either
mechanically or via a bonding material to said structural members.
They can also be placed at a layer boundary in layered structures.
Likewise, they can be inserted into the structure during or after
construction. The devices could also be epoxied to the back of
sheetrock during construction to monitor moisture or temperature of
the inner wall of the medium.
Wireless Embedded Sensor Platforms
The sensor system of the present invention employs a plurality of
wireless embedded sensor platforms strategically or randomly
distributed throughout a medium to be monitored. Each sensor
platform is capable of supporting a plurality of sensor types for
measuring a wide variety of structural, chemical and environmental
conditions associated with the monitored medium. The sensor types
capable of support by the platform include, but are not limited to,
temperature, conductivity, pH, magnetism, noise, pressure, shock,
strain, stress, vibration, etc. Other sensor types not explicitly
recited herein may be used in conjunction with the sensor platform
of the present invention.
Representative of the sensor types used herein are discussed in
Appendix A which is attached hereto. Each of the sensor types
discussed in Appendix A share the common characteristics of being
small, inexpensive, requiring very little power, and are easily
integrated with the sensor platform. Appendix A describes the
operating principle of each sensor and the physical variables they
monitor.
The following is an overview of the mechanical and electrical
features of the wireless embedded sensor platforms employed in the
sensor system of the present invention.
Mechanical Overview
Referring now to FIG. 1, wireless multi-functional sensor platform
100 of the present invention includes at least housing 120 and
sensor electronics 118 enclosed within housing 120, which is
discussed hereinbelow. In general, housing 120 will be formed from
conventional materials known in the art. Preferred materials for
use herein include, but are not limited to, ceramic materials such
as alumina or macor and the like. As one skilled in the art would
readily appreciate, dimensions and configurations for housing 120
can vary accordingly and can be determined on a case-by-case basis.
For example, when housing 120 is disc-like in shape, housing 120
can be 1" in diameter. It is within the contemplation of the
present invention to further reduce the size of housing 120 in
future embodiments to a package volume less than 2.5 cm.sup.3 (0.15
in.sup.3).
In general, sensor housing 120 can include base 114 and cap 116.
Base 114 can be constructed in a two-step process wherein a first
step defines a conventional machining process to acquire the shape
of the base such as the generally disc-like shape as depicted in
FIG. 1. The second step involves defining a firing cycle to enable
base 114 to reach a high mechanical strength. Subsequent to the
second or firing cycle step, the housing material is substantially
identical to production ceramic cast material.
Cap 116 is generally a shape similar to that of base 114 such as a
circular shaped low temperature co-fired ceramic (LTCC) cap 116 as
depicted in FIG. 1. The cap 116 is preferably epoxy-mounted into
the ceramic base 114. However, it is to be understood that other
conventional means for mounting base 114 into cap 116 known to one
skilled in the art can be used herein. In one embodiment, the cap
portion 116 is constructed as comprising several layers 119. The
layers can be of varying or the same thickness, e.g., a thickness
for each layer ranging from about 0.001" to about 0.10" with an
approximate thickness of 0.005" being preferred for each layer for
structural integrity such that the an approximate final thickness
of about 0.040" is achieved. One of the layers of the ceramic cap
116 is designed to function as a patterned transmission antenna.
FIG. 2 illustrates a top view of the patterned transmission antenna
constructed as a layer amongst several layers 119. Other layers
(not shown) that can be included in layers 119 are a spacer layer,
routing layers, power pickup coil layer as are within the purview
of one skilled in the art.
In a preferred embodiment, the housing 120 is made from a
machinable green bisque ceramic of 96% alumina. The green bisque
material was selected as a preferred material based on meeting the
requirements of low cost; requiring only standard automated
machining and; having physical properties which are closest to a
low temperature co-fired ceramic (LTCC) cap 116 which serves as a
substrate upon which the sensor electronics 118 and patterned
antenna 119 are assembled for attachment to base 114. The base 114
can be made from any type of inexpensive ceramic that is capable of
being machined or cast. However, base 114 can also be milled
directly from machinable ceramic, such as Macor, depending upon the
suitability of the intended application.
As stated above, LTCC cap 116 serves as an assembly platform for
the sensor electronics 118 which comprise a number of IC components
attached to LTCC cap 116 using commercial IC packaging and assembly
techniques and materials. In particular, the sensor electronics 118
are attached to LTCC cap 116 with, for example, gold vias and
plating. Once assembled, LTCC cap 116 including the sensor
electronics 118 is attached to the base 114 of sensor housing 120.
LTCC cap 116 is preferably bonded to the base 114 with ceramic
epoxies.
LTCC cap 116 can be a combination of tape dielectric materials with
screen printed thick-film conductor. For example, the tape can be
cut to the desired geometry; vias are mechanically formed where
needed, and conductors are printed on the tape sheets. The various
layers are then stacked and laminated into a monolithic structure
which is dried and fired to produce the desired functional part.
This produces a ceramic laminate, similar to a printed circuit
board, with interlayers of conductor.
FIG. 3 shows a back view of the cap 116 for an application
including a conductivity sensor. In the case where a conductivity
sensor is used, two electrodes 32 and 34 collectively comprise a
single conductivity sensor for making conductivity measurements. As
shown, electrodes 32 and 34 physically protrude through the
exterior or back portion of the cap 116 thereby coming into
physical contact with the medium being monitored or measured (e.g.,
concrete). The conductivity sensor operates by driving a current
out of one electrode 32 and allowing the current to flow through
the medium to be returned to the other electrode 34. A potential is
thus developed by virtue of the current flow. Inside the sensor
platform 100, the potential that develops across the electrodes is
measured, which is a measure of the resisitivity of the medium.
Electrodes 32 and 34 are spaced and sized so that the measured
potential can be transformed directly into a resistance
measurement. Electrodes 32 and 34 may be constructed of palladium
platinum. It is noted that not all sensors are required to be
exposed to the harsh environment, and as such are wholly contained
within the sensor housing 120.
Electronics Overview
Two embodiments of the sensor electronics/circuitry are illustrated
in FIGS. 4a and 4b. In each embodiment, a micro-power processor 45
and associated circuitry provide the system control for a plurality
of sensors types. A unique advantage of the present invention is
that a plurality of sensors types may be attached to the sensor
platform 100 to simultaneously monitor the desired parameters
associated with the condition of the monitored medium. Further,
each sensor platform 100 has a unique identification number that is
recorded during installation for correlating obtained data to
specific location within the monitored medium. In each embodiment,
power is remotely provided to the sensor platform 100 using a
method of near field induction while data is transmitted from the
sensor platform 100 to a field acquisition unit via a radio
frequency (RF) link in a first embodiment, or by means of
absorption modulation, in a second embodiment. Each embodiment is
further described below.
In FIGS. 4a and 4b three sensors 51, 52, 53 are shown. The sensors
51-53 perform the sensing function when power is provided from an
interrogation unit via near field induction. Each sensor 51-53
provides as output, an analog signal to a dedicated channel of
microprocessor 45. The microprocessor 45 may be a microprocessor,
micro-controller, programmable array logic, gate array logic, or
any other chip or circuitry capable of performing the logic and
control functions discussed herein. One type of micro-controller is
MICROCHIP PIC12C509 (Microchip, Chandler, Ariz.), although other
similar micro-controllers can be substituted by those skilled in
the art.
The analog signal obtained as output from each sensor 51-53 is
converted to a digital signal format via an internal
analog-to-digital converter in the microprocessor 45. In alternate
embodiments, the analog-to-digital conversion function can be
performed external to the microprocessor 45 as shown (see External
ADC 47). In addition to performing an analog-to-digital conversion
function, the microprocessor 45 performs the functions of timing,
identification, local data storage, communications protocol
implementation and outputting digital signals to either
transmission VCO circuit (see FIG. 4a) or a modulator 49 (see FIG.
4b).
FIGS. 4a and 4b illustrate alternate embodiments for transmitting
data from the sensor platforms 100 to a field acquisition unit for
receiving the transmitted data. The particular embodiment selected
will depend on the sensor platform's 100 location and the
surrounding environment.
Referring first to FIG. 4a, in a first embodiment, the sensor
electronics 400 of the sensor platform is shown. In the first
embodiment, data is transmitted from the sensor platforms 100 to a
field acquisition unit (not shown) via an RF link, which is well
known in the art. In accordance with a method for using an RF link,
the transmission VCO circuit 49 is embodied as either an RF voltage
or current controlled oscillator to drive the patterned transmit
antenna 119 of FIG. 2. In this case, sensor data outputs of the
microprocessor 45 modulate the transmission VCO circuit 49 using
frequency shift keying FSK, which is well known in the art. The
microprocessor driven transmission VCO circuit 49 then outputs one
of two output frequencies dependent upon the modulation
applied.
Referring now to FIG. 4b, the sensor electronics of FIG. 4b are
similar in most respects to that of FIG. 4a with the following
exceptions. In the second embodiment, data is transmitted from the
sensor platform 100 to a field acquisition unit via absorption
modulation, which is well known in the art. In accordance with this
method, the modulator circuit 49 is embodied as a metal oxide
semiconductor field effect transistor MOSFET or similar device
having comparable switching characteristics. A data signal
corresponding to a binary `1` from the microprocessor 45 causes the
FET to turn on which causes extra or additional loading to be
applied to the sensor platform's 100 pickup coil 46. This
additional loading changes the loading characteristic of the sensor
platform 100 relative to the external interrogation unit's power
field generator. When the FET is turned on, this results in a small
amplitude modulation of the voltage across the induction field
generator coil of the interrogation unit. The resulting amplitude
modulated (AM) signal can be processed at the interrogation unit to
recover the transmitted data. In this case, no transmit antenna is
required. Data is recovered simply by detecting the small amplitude
modulation of the voltage at the interrogator unit (i.e.,
transmitter).
It should also be noted that the migration of the sensor
electronics as described above, to a multi-chip module (MCM)
assembly, chip on board assembly, buried LTCC resistors to further
reduce the size and cost is possible.
One further adaptation contemplated by the present invention
includes the use of a miniature off board oscillator 43, instead of
the microcontroller's 45 on-board oscillator 43 shown, to further
reduce power consumption.
An important technical advantage of the present invention is that
the circuitry of sensor platform 100 may be powered by an induction
field sent by an interrogation unit. Thus, sensor platform 100 does
not need a battery or other local power source or power storage.
Because no battery is needed, sensor platform 100 and associated
circuitry may be placed in the structure to be monitored and
thereafter require little to no maintenance of sensor platform
100.
Turning now to FIG. 5, FIG. 5 is a functional block diagram of an
interrogation unit 500 for powering the sensor platforms 100 and
receiving data there-from. A primary function of the interrogation
unit 500 is to generate a time varying magnetic field, i.e., an
alternating current (AC) induction field to power the plurality of
embedded sensor platforms 100 distributed throughout the medium to
be monitored. This time varying magnetic field is generated by an
AC current flowing in a one-turn coil. This coil on the
interrogation unit 500 is mounted in such a way as to allow it to
be placed directly over where the sensor is embedded. The AC
current flowing in the one-turn coil is generated by an oscillator
502 which provides a stable operating frequency for the power field
generator via crystal control. The output of the oscillator is
amplified by a power field generator 504 which drives the one turn
coil 506. The time varying magnetic field produced by the power
field generator 504 induces an AC voltage in the power pickup coil
46 located inside the sensor platform 100. The induced AC voltage
is rectified in the sensor platform 100 to produce a DC voltage as
an input to the sensor platform's 100 voltage regulator (not
shown). Internal to the sensor platform 100, a zener diode clamps
the regulator's DC input voltage to a safe level. The output of the
voltage regulator then provides power to the sensor platform's 100
remaining sensor electronics.
With continued reference to FIG. 5, there is shown a receiving
antenna 507 for use when the sensor platforms 100 transmit their
data to the interrogation unit 500 via an RF link. In this case, an
RF receiver 509 is used to amplify and filter the RF signal. As
shown, the oscillator 502 provides an input signal to the RF
receiver 509 as part of the signal processing.
In the case where sensor platforms 100 transmit their data to the
interrogation unit 500 via absorption modulation, the absorption
receiver 511 is used. Irrespective of which receiving method is
used, data recovery module 513 is used to provide final signal
conditioning and interfacing to a local computer 515.
The following non-limiting example is illustrative of the
multi-functional sensor system in accordance with the present
disclosure.
EXAMPLE
The operational concept of the multi-functional sensor system of
the present invention is shown in FIG. 6 illustrating an exemplary
application involving bridge monitoring. FIG. 6 illustrates a
bridge structure 62 including a plurality of wireless embedded
sensor platforms 64. For this particular application, the sensor
platforms 64 are placed approximately every two meters throughout
the structural elements of the bridge 62. The locations and
densities of the sensor platforms 64 will depend on the particular
structure being monitored.
As will be discussed below, information from the sensor platforms
64 will be transmitted during interrogation. The interrogation is
performed with an interrogation unit 500 carried on a vehicle 68.
The interrogation unit 500 records data from the sensor platforms
64 as the vehicle 68 passes in proximity to the sensor platforms
64. It should be understood that the interrogation unit 500 need
not be carried on the vehicle 68, and may be hand held or
permanently mounted near the structure to be monitored. With
structures such as bridges, however, mounting the interrogation
unit 500 on a vehicle allows for particularly convenient monitoring
of environmental parameters.
The sensor platforms 64 are powered by near field induction (i.e.,
a power transmitter/reader) 69. As previously mentioned, the sensor
platforms 64 are able to communicate with interrogation unit 500
through the use of radio frequency ("RF") waves or absorption
modulation of the induction power field. With this approach,
wireless, contactless reading of the sensor platforms 64 can be
accomplished. Such communication provides an important technical
advantage of the present invention, since reading of the sensor
platforms 62 may be performed conveniently and quickly.
In operation, circuitry within the interrogation unit 500 generates
an AC magnetic field, to power the sensor platforms 64, and
receives data from the sensor platforms 62 for storage in a local
computer in the interrogation unit 500.
Each of the plurality of sensor platforms 64 has a unique
identification number that is recorded during installation. With
individual ID numbers, the locations of particular sensors are
maintained in a record, and data from those sensors can then be
correlated with their position.
FIG. 7 is an exemplary illustration of a map that is generated as a
product of using the system of the present invention in the
illustrative bridge-modeling example of FIG. 6. As shown in FIG. 7,
the gray-scale map indicates the variation in resistivity values in
the bridge structure. In the present exemplary application, the
resistivity values would be obtained by employing resistivity
sensors in the sensor platforms 64 and recording data from the
resistivity sensors. Such a map would localize areas of suspicion
which would need to be monitored and/or repaired.
It will be understood that various modifications may be made to the
embodiments disclosed herein. Therefore the above description
should not be construed as limiting, but merely as exemplifications
of preferred embodiments. For example, the functions described
above and implemented as the best mode for operating the present
invention are for illustration purposes only. Other arrangements
and methods may be implemented by those skilled in the art without
departing from the scope and spirit of this invention. Moreover,
those skilled in the art will envision other modifications within
the scope and spirit of the claims appended hereto.
APPENDIX A
Resistivity Sensor
The permeability of concrete refers to its ability to transport
moisture, oxygen and chloride ions through concrete to the steel
surface in the concrete. As is known through several studies,
concrete with an electrical resistivity of 120,000 ohm-cm has a low
permeability and low corrosivity whereas concrete with a
resistivity of 10,000 ohm-cm or lower has high corrosivity.
Therefore, electrical resistivity of concrete is a good indicator
of its permeability to the potential corrosion agents. The range of
resistivities expected is about 3 k to about 120 k cm-cm.
The resistivity sensor is based on a conventional four-probe
technique; two probes to inject a current, and two probes to
monitor the potential. In a typical arrangement, all four probes
are equally spaced from each other with the two potential probes
between the two current injection probes. The resistivity is
computed using a modified form of Ohms Law from which the
permeability is estimated. The applied current will be on the order
of 10 microamperes (.mu.A) at less than 1 V, and the total power
requirement will be about 10 .mu.W..backslash.
pH Sensor
It is recommended to measure pH, in addition to chloride ion
concentrations and temperature, to obtain a good estimate of
corrosivity of the concrete environment. Commercial pH meters are
unsuitable for this task because of reliability and size. The
sensor under development is a calorimetric pH sensor, which uses an
LED light source and a photo diode detector. It will measure pH
changes in the 11 to 14 ranges and doesn't require calibration. The
total power requirement for the sensor will be on the order of a
few milliwatts. The wireless multi-functional sensor platform is
being designed to accommodate this sensor.
Chloride Sensor
The chloride sensor includes at least two silver/silver chloride
wires, one freely projecting into the concrete, and the other
interfaced through a ceramic membrane saturated with potassium
chloride (KCl). Each wire is about 1 cm long, and 2 to 5 mm in
diameter. The chloride sensor will be calibrated after fabrication
but will require no calibration at later times. It will measure the
absolute level of chloride concentration in the concrete and
requires very little power to operate.
Temperature Sensor
This sensor is a semiconductor-based sensor. Typically, due to the
slow temperature transients expected inside concrete, the wireless
multi-functional sensor platform should be in equilibrium with the
adjacent concrete temperature. Therefore, the temperature sensor
built into most microcontrollers can be used to sense the local
concrete temperature.
Magnetometer Sensor
This sensor is a miniature sensor, which is sensitive to magnetic
fields produced by currents in the 10.sup.-9 to 10.sup.+1 ampere
range. It requires a few millewatts of power to operate. The
magnetometer is used to sense electrical noise produced by the
pitting corrosion activity and also can be used to measure stray
currents or cathodic protection (CP) current in concrete
structures.
Pitting corrosion is caused by the break down of the protective
oxide film (passive film) on the metal surface. When the film is
broken down (i.e., depassivated), the surface of the metal will
corrode. The corrosion product will generate more of the (i.e.,
passive) metal oxide on the surface, preventing further corrosion.
The corrosion reaction produces a current flow across the
metal/concrete interface. During pitting, the
depassivation-corrosion-repassivation process will repeat randomly,
producing a temporal variation in the corrosion current. These
fluctuations occur at low frequencies (<1 Hz), in the same range
as electronic instrumentation noise or white noise. The noise
associated with pitting corrosion, however, has several unique
characteristics. Most forms of electronic and white noise will have
a single temporal distribution. The electrochemical noise due to
corrosion, on the other hand, has a bimodal distribution. The slope
of the power spectrum plot is also unique for pitting corrosion,
and can be used to indicate its presence.
The magnetic sensor envisioned by the applicants has a wide dynamic
range. It can measure stray currents (.about.10 microamps; 100 nano
Tesla): corrosion current (10 microamps, 100 nano Tesla); CP
current (.about.1 milliamps; 10,000 nano Tesla) in the presence of
a very large earth's magnetic field (50,000 nano Tesla).
It is noted that stray current electrolysis is a source of
corrosion in concrete in industrial areas. Since rebars are buried
in concrete, which is in contact with earth, the steel becomes an
easy carrier of stray current. If the stray current has frequency
components below a few Hz, it can quickly break down the oxide
layer and cause pitting corrosion. Such a low frequency stray
current can originate from light rail systems, high power
industries, and heavy electrical engineering operations. Therefore,
in urban and industrial environments where stray current is common,
a concrete bridge may corrode even in the absence of chloride ions.
The magnetometer sensor will identify the presence of stray current
noise, characterize its frequency and amplitude, and even the
points of stray injection and discharge.
* * * * *